Highly sensitive, selective, and robust sensors capable of monitoring small volumes of body fluids are one of the key components for developing responsive drug delivery systems. Protein engineering and molecular biology have facilitated the molecular design of bio-reagents, which are used as the sensing elements in various systems that offer high selectivity, good response times, and low detection limits. In addition, bio sensors have been developed for physiologically relevant molecules, such as neurotransmitters and hormones.
Stimuli-sensitive hydrogels have found applications in actuators, sensors, drug delivery and bio separations. These materials are able to respond reversibly to an external stimulus that causes a distinct measurable effect on the physical properties of the material. Hydrogels are known to be sensitive to pH, ion concentration, temperature, solvent composition and electric potential. The hydrogels can be also designed to swell upon presence of a target molecule. They can be constructed in a way that the magnitude of swelling can be proportional to the concentration of ligands being present.
Application of a stimuli-sensitive hydrogel in an implantable microscale system for drug delivery is known from US 2004/024832 in which a fluid flow through a channel is regulated by the hydrogel in response to a predetermined stimulus in a medium contacting the hydrogel. The reversible volume change leads to a reversible gating of the channel allowing the microscale component to act as an active microvalve. In this way the hydrogel-actuated microvalve mechanically couples the volume change of the hydrogel in response to a predetermined chemical compound to the opening and closing of the channel by a membrane.
A disadvantage of the known system is that is only capable of drug delivery and that is not capable of accurate monitoring of physiological parameters outside of and/or within a bodily lumen.